blocks.py

"""GPT Blocks used for the GPT Model."""
from typing import Dict, Optional, Tuple
import torch
import torch.nn as nn
from .attention import ATTN_CLASS_REGISTRY
from .norm import NORM_CLASS_REGISTRY


class MPTMLP(nn.Module):
    def __init__(
        self, d_model: int, expansion_ratio: int, device: Optional[str] = None
    ):
        super().__init__()
        self.up_proj = nn.Linear(d_model, expansion_ratio * d_model, device=device)
        self.act = nn.GELU(approximate="none")
        self.down_proj = nn.Linear(expansion_ratio * d_model, d_model, device=device)
        self.down_proj._is_residual = True

    def forward(self, x):
        return self.down_proj(self.act(self.up_proj(x)))


class MPTBlock(nn.Module):
    def __init__(
        self,
        d_model: int,
        n_heads: int,
        expansion_ratio: int,
        attn_config: Dict = {
            "attn_type": "multihead_attention",
            "attn_pdrop": 0.0,
            "attn_impl": "triton",
            "qk_ln": False,
            "clip_qkv": None,
            "softmax_scale": None,
            "prefix_lm": False,
            "attn_uses_sequence_id": False,
            "alibi": False,
            "alibi_bias_max": 8,
        },
        resid_pdrop: float = 0.0,
        norm_type: str = "low_precision_layernorm",
        device: Optional[str] = None,
        **kwargs
    ):
        del kwargs
        super().__init__()
        norm_class = NORM_CLASS_REGISTRY[norm_type.lower()]
        attn_class = ATTN_CLASS_REGISTRY[attn_config["attn_type"]]
        self.norm_1 = norm_class(d_model, device=device)
        self.attn = attn_class(
            attn_impl=attn_config["attn_impl"],
            clip_qkv=attn_config["clip_qkv"],
            qk_ln=attn_config["qk_ln"],
            softmax_scale=attn_config["softmax_scale"],
            attn_pdrop=attn_config["attn_pdrop"],
            d_model=d_model,
            n_heads=n_heads,
            device=device,
        )
        self.norm_2 = norm_class(d_model, device=device)
        self.ffn = MPTMLP(
            d_model=d_model, expansion_ratio=expansion_ratio, device=device
        )
        self.resid_attn_dropout = nn.Dropout(resid_pdrop)
        self.resid_ffn_dropout = nn.Dropout(resid_pdrop)

    def forward(
        self,
        x: torch.Tensor,
        past_key_value: Optional[Tuple[torch.Tensor]] = None,
        attn_bias: Optional[torch.Tensor] = None,
        attention_mask: Optional[torch.ByteTensor] = None,
        is_causal: bool = True,
    ) -> Tuple[torch.Tensor, Optional[Tuple[torch.Tensor]]]:
        a = self.norm_1(x)
        (b, _, past_key_value) = self.attn(
            a,
            past_key_value=past_key_value,
            attn_bias=attn_bias,
            attention_mask=attention_mask,
            is_causal=is_causal,
        )
        x = x + self.resid_attn_dropout(b)
        m = self.norm_2(x)
        n = self.ffn(m)
        x = x + self.resid_ffn_dropout(n)
        return (x, past_key_value)